Is there a way to propagate changes from the foreign data source through the FDW to Postgres?
Or would it just be some kind of task polls the foreign data source pulling a delta?
There's no facility for deltas, or any other way of propagating changes.
Edit: And to answer your original question, yes you can absolutely base the materialized view on data obtained from a foreign table.
Once I heard those words mentioned, I began to imagine what sort of interaction with a foreign data source's operations log FDW might have.
Of course, and depending on the foreign data source, you could probably even expose an operations log to Postgres via a FDW by defining it as a foreign table... Effectively opening up the possibility of eventually consistent replication. Lots of data stores' operations logs are accessible data collections/tables anyways.
All of this is exciting stuff!